Method for actuating an electromechanical parking brake device

ABSTRACT

A method for actuating an electromechanical parking brake device for a brake that can be actuated by an electromechanical actuator comprised of an electric motor and of a reduction gear connected downstream of the electric motor and provided for converting a rotational motion into a translatory motion. In order to guarantee that the electromechanical parking brake device works reliably in all operating states without using a tension force sensor, the method determines and stores, during the actuation of a parking brake device, a mean value of the torque of the electric motor necessary for generating the application force of the brake corresponding to the parking brake actuation and simultaneously determines the actuator position and actuates the electric motor at later points of time in such a way that it generates this torque multiplied by a correction factor kη=&gt;1 so that the exerted tension force is maintained or increased.

TECHNICAL FIELD

The present invention generally relates to a method for actuatingelectromechanical parking brake devices and more particularly relates toa method for actuating an electromechanical parking brake device for abrake that can be actuated by means of an electromechanical actuator.

BACKGROUND OF THE INVENTION

An electromechanically operable brake of this type is disclosed ininternational patent application WO 99/45292. The electromechanicalparking brake device of the referenced brake consists of a detent pawlthat is operable by means of an electromagnet and can be put intoengagement with a gear rim fastened at the rotor of the electric motor.However, said publication does not give any hints with regard to theactuation of the parking brake device. The prior art brake is notprovided with a sensor for detecting the tension force so that saidforce has to be estimated on the basis of other data.

BRIEF SUMMARY OF THE INVENTION

Therefore it is an object of the present invention to disclose a methodfor actuating an electromechanical parking brake device guaranteeing areliable parking brake function in all operating states without using atension force sensor.

This object is achieved according to the invention in that during theactivation of the parking brake device a mean value of the torque of theelectric motor, which is required for exerting the application force ofthe brake corresponding to the parking brake operation, is determinedand stored while the actuator position is simultaneously detected, andthe electric motor is actuated at later points of time in such a fashionthat it generates said torque which is multiplied by a correction factorkη=>1 in order to maintain or increase the exerted tension force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents an embodiment of a brake, which can be actuatedelectromechanically and is provided with an electromechanical parkingbrake device, and which allows implementing the method according to theinvention can be applied, in axial cross-section,

FIG. 2 represents the embodiment of the parking brake device used withthe brake according to FIG. 1,

FIG. 3 represents the parking brake device according to FIG. 2 in itsinitial position in broken-up representation,

FIG. 4 shows the parking brake device according to FIG. 2 in actuatedposition in broken-up representation, and

FIG. 5 is a flow chart showing the method according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The brake that can be actuated in an electromechanical manner, isrepresented in particular in FIG. 1, where the method according to theinvention is applicable and which is designed as a floating-caliper diskbrake, is principally comprised of an electromechanical actuator and abrake caliper which is indicated only schematically and is slideablyarranged on a fixed support (not shown). A pair of friction linings 4and 5 is arranged in the brake caliper in such a way that they arefacing the left and the right side of a brake disc 6.

In the following, the friction lining 4 shown at the right in thedrawing is indicated as first friction lining and the other frictionlining indicated with reference numeral S is defined as second frictionlining. While the actuator can bring the first friction lining 4 intodirect engagement with brake disc 6 by means of an actuating element 7,through the actuator, the second friction lining 5 is pressed againstthe opposite lateral face of the brake disc 6 by the effect of areaction force of the brake caliper caused by the actuation of theassembly. The actuator, which is mounted on the brake caliper by way offastening means (not shown), has a modular design and consists inprinciple of three subassemblies or modules, respectively, which can behandled independently of each other, i.e. a drive unit 1, a reductiongear 2 actuating the first friction lining 4 and a second reduction gear3 inserted, in terms of effect, between the drive unit 1 and the firstreduction gear 2.

The above-mentioned drive unit 1 consists of an electric motor 10provided as a motor which in the example is excited by a permanentmagnet and electronically commutated, the stator 9 of which is arrangedin an immovable manner in a motor housing 8 and the rotor 11 of which isformed by a ring-type support 13 supporting several segments ofpermanent magnet 14. The first reduction gear 2 is inserted, in terms ofeffect, between the electric motor 10 and said actuating element 7, thereduction gear being designed as a ball screw 16 to 21 supported in agear housing 15 which can also be designed in one part with theabove-mentioned brake caliper. The ball screw consists herein of athreaded nut 16 and a threaded spindle 17, several balls 18 beingarranged between the threaded nut 16 and the threaded spindle 17 andrevolving during the rotational motion of the threaded spindle 17, thuscausing the threaded nut 16 to execute an axial or a translatory motion,respectively. The threaded nut 16 preferably forms said actuatingelement 7. The threaded spindle 17 driven by the electric motor 10 bymeans of the second reduction gear 3, is formed preferably of threeparts consisting of a tubular first spindle part 19 which is inengagement with the threaded nut 16 by means of said balls 18, aring-type second spindle part 20 as well as a third spindle part 21.

The arrangement is preferably made in such a way that the rotor 11 ofmotor 10 drives the third spindle part 21 by inserting the secondreduction gear 3 while the threaded nut 16 is supported on the firstfriction lining 4.

In the embodiment described in the drawing, a reduction of the requiredmotor torque is achieved by means of a suitable integration of a planetgear 30-34 forming the second reduction gear 3 mentioned above. Theplanet gear arranged, in terms of effect, between the rotor 11 and thethreaded spindle 17, consists of a sun wheel 30 preferably formed by anexternally toothed area 22 on the rotor 11, several stepped planetwheels two of which are represented and indicated with referencenumerals 31 and 32, as well as an internally toothed ring 33. Thestepped planet wheels 31, 32 mounted in a planet cage 34 are providedwith a first stage cooperating with the sun wheel 30 as well as a secondstage cooperating with the ring gear 33, the first stage being formed bytoothed wheels 31 a, 32 a of a larger diameter and the second stagebeing formed by toothed wheels 31 b, 32 b of a smaller diameter. Saidplanet cage 34 is preferably designed in such a way that the areabetween the supporting points of the planet wheels 31, 32 and theconnecting point of the threaded spindle 17 allows a small axial as wellas a radial play and a small offset angle and is formed, e.g. as alamellar disc or a bellow. An internally toothed area of a cover 23forming the housing of the planet gear forms the internally toothed ring33.

The above-mentioned threaded nut 16 of the ball screw is guided orsupported in a bowl-type guide member 12. The threaded nut 16 issupported in the guide member 12 in the area facing the first frictionlining 4 by means of a first sliding ring 28 arranged in the guidemember 12 as well as in the final area remote from the friction lining 4by means of a second friction ring 29 arranged on the threaded nut 16.

Furthermore, it can be taken from FIG. 1 that the second ring-typespindle element 20 is supported on a thrust bearing 26 arranged withinthe guide member 12 while the third spindle element 21 is connected withthe planet cage 34 of the second reduction gear 3 by means of aform-locking plug-in connection. To this effect, the end of the thirdspindle element 21 is e.g. formed as component of a torx connection oras hexagon, which is pushed into a suitable opening in the planet cage34. In this case it is particularly favorable if the form-lockingplug-in connection is coupled in a torsion-proof, radially yielding andflexible manner to the planet cage 34. The coupling is achieved by meansof an external ring 51 of a radial bearing 50 provided in the cover 23.An elastic seal or sealing collar 27 clamped between the threaded nut 16and the guide member 12 prevents the ingress of dirt into the interiorof the ball screw.

Furthermore, for a perfect function of the actuation unit according tothe invention it is useful that the threaded nut 16 is provided with anaxial projection (not shown) on its end remote from the friction lining4 cooperating with a stop portion formed at the circumference of thesecond spindle element 20 during the resetting process. By supporting alateral surface of the projection on the stop portion, a further resetof the threaded nut 16 is reliably prevented so that the two parts 16,20 do not get jammed.

In order to detect the actual position of the rotor 11, a positiondetection system 46 is provided which is not shown in detail. Theposition information is then determined by means of a Hall sensor or amagnetoresistive element.

In order to be able to realize the function of a parking brake, theactuation unit according to the present invention is provided withelectromechanical means (see FIG. 2) allowing in cooperation with therotor 11 of the electric motor 10 the locking of the latter. In theembodiment shown the electromechanical means is formed by means of anelectromechanically actuated freewheel designated by reference numeral35, cooperating with a radial bearing 24 in which the rotor 11 issupported. The electrical actuator associated with the freewheel 35 hasthe form of a mechanical flip-flop, the state of which changes with eachshort energization.

As can be taken in particular from FIG. 2 to 4, essential parts of thefreewheel 35 are integrated in said radial bearing 24. To this end, theouter ring 36 as well as the inner ring 37 of the radial bearing 24 areextended on one side in such a way that they delimit a ring areaaccommodating a clamping member 38, thus guaranteeing a form-lockingconnection between the bearing rings 36, 37 and the clamping member 38by the particular configuration of the extended area of the bearingrings 36, 37. The outer ring 36 is preferably provided with a radialrecess 39 in its area cooperating with the clamping member 38, therecess being limited on one side by an inclination or ramp 40, while theinner ring 37 is provided with a contour 41 corresponding to the contourof the clamping member 38 and limiting a clamping gap together with therecess 39. The clamping member 38, which can be designed as a clampingroll or in the form of a ball, is pretensioned by means of a ring-typespring member 42 towards said recess 39.

An electromagnetic actuation unit is used for actuating the freewheel35, being designated by the reference numeral 43 in the exampleillustrated. The actuation unit 43 essentially consists of a bistableelectromagnet 44 as well as a tappet 45 cooperating with the armature ofthe electromagnet 44 displacing the clamping member 38 radially whenactivating the electromagnet 44. The tappet 45 is guided in a tubularguide member 47 formed on a ring-type accommodating member 48 arrangedin the motor housing 8 and accommodating the bearing outer ring 36.

When activating the parking brake device 35, the following functionalorder is provided and described in detail with regard to FIG. 5:

First the electromechanical brake is applied by corresponding operationof the electric motor 10 until the necessary application force levelF_(park) is reached. The brake is assumed to be in “warm” condition sothat the activation of the parking brake device is realized according tothe characteristic F-f(φ). The application force F_(park) to be adjustedis achieved with an actuator position or the rotor position defined withφ_(park). In point A defined by the coordinates φ_(park), F_(park) theparking brake device 35 is locked. At the same time the mean valueM_(park) of the torque produced by the electric motor 10 is determinedby measuring the current supplied to the electric motor 10 correspondingto the efficiency=1 of the assembly. The mean value is preferablyexamined with regard to a lower limit value. If the mean value of thetorque is below said limit value, the torque (and thus also theapplication force) is increased up to this limit value. If the meanvalue of the torque is above said limit value, the application forceadjusted by the control is maintained. The mean value of the torqueapplied to the brake is stored in a non-volatile memory (EEPROM). Thenthe mean torque value M_(park) is multiplied by a correction factor kη>1in an electronic control unit (not shown), thus defining a higher torquevalue M_(1park). Under the assumption that in this case the ascendingbranch of the characteristic hysteresis curve depending on theefficiency has to be considered or is used, a higher application forceF₁ results from the higher torque value M_(1park) which corresponds to alocking point A₁ on the characteristic curve F=f(φ). A changed actuatorposition φ_(1park) corresponds to the additional operation of theactuator according to the mentioned application force increase.

It has to be mentioned that the rotor 11 or the bearing inner ring 37,respectively, is displaced against the clamping direction of thefreewheel 35, i.e. to the left in the drawing, when the brake isapplied. When the clamping member 38 is displaced towards the countour41 by activating the electromagnet 44, when the parking brake isactuated, the clamping member rolls towards the tapering clamping gap onthe above-mentioned ramp 40. If the current supplied to the electricmotor 10 is reduced, the spring force of the applied brake tries to turnthe rotor 11 or the bearing inner ring 37 towards the clampingdirection. Thus, the parking brake device is securely locked. The lockedposition of the parking brake device is represented in FIG. 4.

If the parking brake device is released after a short period of time(while a brake re-application has not yet taken place) the electricmotor 10 has to continue for a defined period of time to apply the brakeby defining a torque value M_(rel)=k_(rel)*k_(nη)*M_(park) exceedingconsiderably the torque M_(park) defined before and the electromagnet 44has to be actuated once in order again to move the tappet 45 upwards.The clamping member 38 relieved hereby is pressed into the recess 39 ofthe bearing outer ring 36 by the force of the spring element 42 by whichit is pretensioned and the rotor 11 can freely turn in both directions.A release of the parking brake device can alternatively also be achievedby that the electric motor is operated for a predefined period of timein such a way that it generates its maximum torque M_(max).

The procedure is similar in the case in which the preset applicationforce is reduced to a value denominated F_(2park) due to cooling down ofthe brake with unchanged actuator position φ_(park), with the parkingbrake device being locked in point A. Equivalent to this value is anoperating state represented by a point A₂ on a characteristic curveF_(T)=f(φ) valid for the cooled condition of the brake and caused by atemperature-dependent displacement of the above-mentioned characteristiccurve. The repeated actuation of the electric motor 10 necessary forincreasing the required application force to said value F₁, isrepresented by the coordinate φ′_(park).

Within the scope of the present invention, of course severalmodifications of the method claimed are also possible. Hence, the methodcan be repeated in previously defined periods of time after the firstactivation of the parking brake device, if necessary, depending on atemperature difference of the actuator. Here, the actuator temperaturedifference corresponds preferably to the difference between the actuatortemperature during the first activation of the parking brake device orduring the last re-application of the brake and the actual actuatortemperature estimated e.g. by means of an actuator temperature model.Said correction factor kη depends on the actuator efficiency or on ameasured or estimated inclined position of the vehicle, respectively.Besides, it is possible that during the release operation of the parkingbrake after a further necessary application of the brake a newcharacteristic curve for actuator position and application force of theactuator is estimated.

1. Method for actuating an electromechanical parking brake device for abrake that can be actuated by means of an electromechanical actuator, inwhich the actuator is comprised of an electric motor and a reductiongear that is connected downstream of the electric motor and is providedfor converting a rotational motion into a translatory motion, and theelectromechanical parking brake device is provided in the form of alocking mechanism which can prevent the rotational motion of theactuator in the direction of release and which can only be releasedagain by further application, wherein during the actuation of theparking brake device, a mean value M_(park) of the torque of theelectric motor, which is required for exerting the application force ofthe brake corresponding to the application of the parking brake, isdetermined and stored while the actuator position (φ) is simultaneouslydetected, and the electric motor is actuated at later points in time insuch a fashion that it generates said torque M_(park) that is multipliedby a correction factor kη=>1 in order to maintain or increase theexerted tension force, wherein the determined mean torque value (Mpark)is controlled with regard to a lower limit value and is set to thislower limit value when it falls below this limit value.
 2. Methodaccording to claim 1, wherein the detection of the torque (M_(park)) isachieved by measuring the current supplied to the electric motor. 3.Method according to claim 1, wherein the electric motor after the firstactuation of the parking brake device is actuated in previously definedperiods of time in such a way that it generates the torque (M_(park))multiplied by the correction factor kη=>1.
 4. Method according to claim1, wherein the electric motor is actuated depending on an actuatortemperature difference in such a way that it generates the torque(M_(park)) multiplied with the correction factor kη=>1.
 5. Methodaccording to claim 4, wherein the actuator temperature is estimated bymeans of an actuator temperature model.
 6. Method according to claim 1,wherein the correction factor kη depends on the actuator efficiency. 7.Method according to claim 1, wherein the correction factor kη depends ona measured or estimated inclined position of the vehicle.
 8. Methodaccording to claim 1, wherein during releasing the parking brake devicea new characteristic curve for actuator position—application force isestimated.
 9. Method for actuating an electromechanical parking brakedevice for a brake that can be actuated by means of an electromechanicalactuator, in which the actuator is comprised of an electric motor and areduction gear that is connected downstream of the electric motor and isprovided for converting a rotational motion into a translatory motion,and the electromechanical parking brake device is provided in the formof a locking mechanism which can prevent the rotational motion of theactuator in the direction of release and which can only be releasedagain by further application, wherein during the actuation of theparking brake device, a mean value M_(park) of the torque of theelectric motor, which is required for exerting the application force ofthe brake corresponding to the application of the parking brake, isdetermined and stored while the actuator position (φ) is simultaneouslydetected, and the electric motor is actuated at later points in time insuch a fashion that it generates said torque M_(park) that is multipliedby a correction factor kη=>1 in order to maintain or increase theexerted tension force, wherein the actuator temperature differencecorresponds to the difference between the actuator temperature duringthe first actuation of the parking brake device or the last operationthe electric motor, respectively, for generating the torque (M_(park))multiplied by the correction factor kη=>1 and the actual actuatortemperature.
 10. Method for actuating an electromechanical parking brakedevice for a brake that can be actuated by means of an electromechanicalactuator, in which the actuator is comprised of an electric motor and areduction gear that is connected downstream of the electric motor and isprovided for converting a rotational motion into a translatory motion,and the electromechanical parking brake device is provided in the formof a locking mechanism which can prevent the rotational motion of theactuator in the direction of release and which can only be releasedagain by further application, wherein during the actuation of theparking brake device, a mean value M_(park) of the torque of theelectric motor, which is required for exerting the application force ofthe brake corresponding to the application of the parking brake, isdetermined and stored while the actuator position (φ) is simultaneouslydetected, and the electric motor is actuated at later points in time insuch a fashion that it generates said torque M_(park) that is multipliedby a correction factor kη=>1 in order to maintain or increase theexerted tension force, wherein for releasing the parking brake devicethe electric motor is actuated for a predefined period of time in such away that a torque specification M_(rel)=k_(rel)*k_(η)*M_(park) issatisfied.
 11. Method for actuating an electromechanical parking brakedevice for a brake that can be actuated by means of an electromechanicalactuator, in which the actuator is comprised of an electric motor and areduction gear that is connected downstream of the electric motor and isprovided for converting a rotational motion into a translatory motion,and the electromechanical parking brake device is provided in the formof a locking mechanism which can prevent the rotational motion of theactuator in the direction of release and which can only be releasedagain by further application, wherein during the actuation of theparking brake device, a mean value M_(park) of the torque of theelectric motor, which is required for exerting the application force ofthe brake corresponding to the application of the parking brake, isdetermined and stored while the actuator position (φ) is simultaneouslydetected, and the electric motor is actuated at later points in time insuch a fashion that it generates said torque M_(park) that is multipliedby a correction factor kη=>1 in order to maintain or increase theexerted tension force, wherein for releasing the parking brake devicethe electric motor is operated for a predefined period of time in such away that its maximum torque M_(rel)=M_(max) is generated.